This invention relates generally to apparatus and methods for controlling the polarization of light. This invention relates particularly to apparatus and methods for controlling the polarization of light propagating in an optical fiber. Still more particularly, this invention relates to apparatus and methods for controlling the polarization of light in fiber optic rotation sensing systems and in coherent communications systems.
A fiber optic ring interferometer typically comprises a loop of fiber optic material having counter-propagating light waves therein. After traversing the loop, the counter-propagating waves are combined so that they constructively or destructively interfere to form an optical output signal. The intensity of the opitcal output signal varies as a function of the interference, which is dependent upon the relative phase of the counter-propagating waves.
Fiber optic ring interferometers have proven to be particularly useful for rotation sensing. Rotation of the loop creates a relative or non-reciprocal phase difference between the counter-propagating waves, in accordance with the well known Sagnac effect, with the amount of phase difference being a function of the angular velocity of the loop about its sensing axis. The optical output signal produced by the interference of the counter-propagating waves varies in intensity as a function of the rotation rate of the loop. Rotation sensing is accomplished by detecting the optical output signal and processing the optical output signal to determine the rotation rate.
Some familiarity with polarization of light and propagation of light within an optical waveguiding structure will facilitate an understanding of the present invention. It is well-known that a light wave may be represented by a time-varying electromagnetic field comprising orthogonal electric and magnetic field vectors having a frequency equal to the frequency of the light wave.
An electromagnetic wave propagating through a guiding structure can be described by a set of normal modes. The normal modes are the permissible distributions of the electric and magnetic fields within the guiding structure, for example, a fiber optic waveguide. The field distributions are directly related to the distribution of energy within the guiding structure. The normal modes are generally represented by mathematical functions that describe the field components in the wave in terms of the frequency and spatial distribution in the guiding structure. The specific functions that describe the normal modes of a waveguide depend upon the geometry of the waveguide. For an optical fiber, where the guided wave is confined to a structure having a circular cross section of fixed dimensions, only field having certain frequencies and spatial distributions will propagate without severe attenuation. The waves having field components that propagate unattenuated are called normal modes. A single mode fiber will propagate only one spatial distribution of energy, that is, one normal mode, for a signal of a given frequency.
In describing the normal modes, it is convenient to refer to the direction of the electric and magnetic fields relative to the direction of propagation of the wave. The direction of the electric field vector in an electromagnetic wave is the polarization of the wave. In general, a wave will have random polarization in which there is a uniform distribution of electric field vectors pointing in all directions permissible for a given mode. If all the electric field in a wave points in only a particular direction, the wave is linearly polarized. If the electric field consists of two orthogonal electric field components of equal magnitude, the electric field is circularly polarized because the net electric field is a vector that rotates around the optic axis at an angular velocity equal to the frequency of the wave. If the two linear polarizations are unequal, the wave has elliptical polarization. In general, any arbitrary polarization can be represented by the sum of two orthogonal linear polarizations, two oppositely directed circular polarizations or two counter rotating elliptical polarizations that have orthogonal major axes.
The boundary between the core and cladding of an optical fiber is a dielectric interface at which certain well-known boundary conditions on the field components must be satisfied. For example, the component of the electric field perpendicular to the interface must be continuous. A single mode optical fiber propagates electromagnetic energy having an electric field component perpendicular to the core-cladding interface. Since the fiber core has an index of refraction greater than that of the cladding and light impinges upon the interface at angles greater than or equal to the critical angle, essentially all of the electric field remains in the core by internal reflection at the interface. To satisfy both the continuity and internal reflection requirements, the radial electric field component in the cladding must be a rapidly decaying exponential function. The exponentially decaying field is usually called the "evanescent field."
The velocity of an optical signal depends upon the index of refraction of the medium through which the light propagates. Certain materials have different refractive indices for different polarizations. A material that has two refractive indices is said to be birefringent. A standard single mode optical fiber may be regarded as a two mode fiber because it will propagate two waves of the same frequency and spatial distribution that have two different polarizations. Two different polarization components of the same normal mode can propagate through a briefringent material unchanged except for a velocity difference between the two polarizations.
It is well known that in many fiber optic systems it may be desirable to have light of a known polarization state at selected points for input to components whose operation is polarization dependent in order to minimize errors. The state of polarization is particularly important in a device such as a fiber optic rotation sensor. In a polarized optical fiber rotation sensing system, drift errors due to changes in polarization are determined by the quality of the polarizer.
The polarization state of light propagating in single mode optical fiber is not stable with time or distance along the fiber. In a fiber fiber optic rotation sensor that includes a polarizer, a preferred polarization state is defined at the location of the polarizer before the optical signal is split to form the counterpropagating waves that are input to the sensing loop. The two counterpropagating waves returning from the sensing loop to the polarizer must have polarization states that match the originally defined polarization state.
However, the birefringence of normal optical fiber will in general cause a polarization mismatch between the input and output waves. This polarization mismatch will result in several undesirable conditions, the most important of which include reduced signal strength and increased parasitic non-reciprocal signal at the detector. Furthermore, the birefringence of the fiber changes with time, for example due to temperature changes, acoustic fluctuations, mechanical deformations such as bending, twisting, squeezing or kinking of the fiber, and magnetic field fluctuations. The resulting time dependent polarization mismatch leads to an unstable signal strength and drift of the non-reciprocal signal component, which is used to indicate the rotation rate. Both the static and time dependent effects of mismatch in the polarization state degrade the performance of the fiber optic rotation sensor.
A linear polarization state in an optical fiber is typically achieved with some type of linear polarizer such as the fiber optic polarizer described in U.S. Pat. No. 4,386,822 to Bergh. The polarization state input to the polarizer is arbitrary in general. The polarizer couples light of undesired polarizations out of the fiber and permits light having only a selected desired polarization to propagate through the fiber. Bergh discloses a fiber optic polarizer including a length of optical fiber mounted in a curved groove in a quartz substrate. The substrate and a portion of the optical fiber are ground and polished to remove a portion of the cladding from the fiber to form an interaction region. The portion of the fiber in the groove is convexly curved as viewed looking toward the polished surface. The birefringent crystal is mounted on the substrate over the interaction region in close proximity to the core of the fiber optic material. The crystal is positioned to partially intersect the path of light propagating in the core of the optical fiber so that evanescent field coupling couples light of undesired polarizations from the optical fiber into the crystal.
In previous systems for active polarization control in fiber optic rotation sensors it has been necessary to utilize polarizers that pass light having the desired polarization while rejecting unwanted polarization components in a direction non-collinear with the output fiber. In such systems the desired polarization state is maintained by adjusting the birefringence of the fiber through a polarization control device to a state that minimizes the intensity of the ejected non-collinear output. To maintain the required polarization state, a compensating birefringence effect must be induced in the sensing loop of the fiber optic rotation sensor. This prior technique requires a complex polarizer, extra photodetection equipment and complex electronic control circuitry for providing the required feedback signals.
Polarization control is also utilized in coherent fiber optic communications systems in which the carrier signal is derived from a long coherence length solid state laser. The signal is transmitted as a modulation applied as amplitude, frequency or phase variations in the optical carrier. In an optical heterodyne receiver, the light from the transmission fiber and the light from a local oscillator laser are mixed to generate an intermediate frequency that typically falls in the microwave range. Standard microwave techniques are then used to demodulate the intermediate frequency signal.
The polarization states of the two interfering light waves must be matched at the mixer in order to maintain the optimal signal sensitivity. If ordinary non-polarization preserving single mode optical fiber is used as the transmission medium, then the birefringence present in such fiber will in general give rise to a mismatch in polarization state between the two interfering light waves at the photodetector. The amount of the mismatch will be unstable with time for the reasons described above in the case of the fiber optic rotation sensor. To maintain the required polarization state match a compensating birefringence effect must be induced in the local oscillator or in the signal arms of the receiver. The active polarization control system used in the fiber optic rotation sensor can be used to provide the compensating birefringence.
The prior polarization control techniques when used in a coherent fiber optic communications system require an active polarization controller in each fiber arm that goes into the 3 dB coupler and one or two polarizers depending upon whether or not the balanced mixer approach is used. The polarizers could alternatively be replaced by a polarization preserving coupler. Another method for overcoming the polarization state mismatch utilizes a polarization insensitive receiver, a bulk optical polarizing beam splitter and two sets of detector electronics. If the balanced mixer approach is used then the number of components doubles. All of these prior art polarization control techniques have excessive numbers of components, complexity and high cost.
Mohr, F. A. and Scholz, U. "Polarization Control for an Optical Fiber Gyroscope", Fiber Optic Rotation and Related Technology, Springer Verlag, 1982, pp. 163-168 describes a bulk optics implementation of a system for propagating an optical signal of a selected polarization in an optical fiber. The apparatus includes an optical fiber, a polarizing beam splitter for providing an optical output signal from the optical fiber, a photodetector, a pair of PZT fiber squeezers, and feedback electronics. The polarizing beam splitter takes light of both the desired polarization and the undesired polarization from the optical fiber. After the signal taken from the fiber has been polarized, it impinges upon the photodetector, which produces an electrical error signal indicative of the undesired polarization. The feedback electronics includes a pair of proportional integral controller circuits that drive the PZT fiber squeezers. The controller circuits are modulated with quadrature signals from a quadrature oscillator that produces two oscillatory signals that are .pi./2 out of phase.
U.S. Pat. No. 4,753,507 to DePaula, et al. discloses a fiber squeezer including a frame that applies a preload to an optical fiber to permit variation of the birefringence by either increasing or decreasing the preload. The optical fiber and a piezoelectric transducer are retained in the frame, and a voltage source is connected to the piezoelectric transducer to control the force on the fiber, which controls the refractive indices of the fiber by means of the photoelastic effect. DePaula et al. also disclose three fiber squeezers arranged in a line along the length of an optical fiber to adjust the polarization of light guided by the fiber.
U.S. Pat. Nos. 4,729,622 to Pavlath, 4,725,113 to Chang et al. and 4,695,123 all disclose optical fiber polarization control systems that include a polarizer and a system of fiber squeezers. The polarizer couples from the optical fiber light of the undesired polarization. The light coupled out of the fiber impinges upon a photodetector, which forms an electrical signal used to control the fiber squeezers. The fiber squeezers are actuated to provide the polarization input to the polarizer that minimizes the intensity of the light coupled from the fiber.
U.S. Pat. No. 4,389,090 to LeFevre discloses an optical fiber polarization controller that includes portions of the fiber wound around three spools. The spools are rotatable on a common axis to adjust the polarization of the light guided by the fiber.